HF and VHF. Which do we use, and when?

[z.khalid]

I can't seem to find out, as Pilots which one we usually use?
What are the major differences?

Can someone please explain this to me..
Thank you.

[mutt]

Usually you will use VHF, but this has a limited range, when you are outside this range you have the option of using HF, i.e. when flying across the Atlantic.

Mutt

[baobab72]

hf notes

hi there

i will copy and paste my notes about the HF.
bear in mind thatnormally when you fly in those areas where VHF is not available due to limted coverage, the ATC will direct you to contact the next secor on the HF.

HF radio communications are used in those areas where VHF radio communications are not available due to their limited signal coverage: as matter of fact HF radio signals, as opposed to VHF radio signals which propagate via direct ground waves, use as preferred path of propagation sky-waves reflected back by the ionosphere.
The signal coverage and thus the distance at which the signal can be received depends upon the reflectivity of the ionosphere, upon the impact angle and upon the frequency.
The ionosphere is a layer stretching from about 50km above the earth surface up to 500km, where due to the action of the ultra-violet radiations from the sun the negatively charged electrons become separated from their atoms which acquire a positive charge: when a radio signal enters the ionosphere it experiences refraction which is the path alteration occurring as a radio signal travels through mediums of different density and molecular properties - resulting into its path being deflected from its straight line, bouncing back towards the earth.
The magnitude of the refraction and the distance at which the signal can be received, depend upon the reflectivity of the ionosphere and upon its thickness affected by the amount of ultra violet solar radiations available and thus by to the time of the day - during the day time when solar radiations are abundant up to four different layers referred to as D, E, F1 and F2 in ascending order of height can be counted while at nite as the solar radiations attenuate the number of layers is reduced to two - D and F -; by the time of the year determining the distance of the earth from the sun and by other cosmic factors such as the sunspots or solar flares.
The amount of energy reflected back to earth is also determined by the impact angle defined by the vertical and the propagation path: the angle producing the first sky wave is referred to as the critical angle and its distance from the transmitter represents the skip distance.
An angle steeper than the critical one will result into a large amount of energy travelling through the ionosphere suffering too much attenuation while a shallower angle will result into an increase in the skip distance and thus into an increase in the so called dead zone where the is no radio coverage.
The critical angle is determined amongst other factors by the frequency, the higher the frequency the larger the critical angle and the greater is the skip distance; in addition the higher the frequency the greater is the degree of ionization required to reflect the signal back to earth.
The ideal frequency will then be a frequency referred to as MUF or Maximum Useable Frequency, that will keep the skip distance just short of the receiver: a higher frequency than the MUF will result into an excessive critical angle and thus into an excessive skip distance putting the receiver within the dead zone, while a lower frequency will cause the signal to suffer excessive attenuation.
The MUF is also affected by the time of the day, as matter of fact at nite when the E layer is at its weakest and the signal has to travel all the way up to the F layer to find a similar degree of reflectivity, in order to obtain the same skip distance as during the day a lower frequency must be used.
An altered or modulated signal such a radio signal is comprised of a carrier wave – the original signal - and of two side bands referred to as the upper side band or USB and the lower side band or LSB. The transmitter to limit the amount of power required to broadcast the signal and thus increasing its range will attenuate the carrier wave together with one side band, focusing all the power available to transmit the remaining side band. For aeronautical HF communications normally only the USB is used.



also watch out for those pilot eaters out there ready to launch at you, as soon as you as a sort of obvious question!!!

[CJ Driver]

Although you have received two posts on the benefits and technical aspects of HF, the abbreviated answer is we (almost) all use VHF all the time. Most aircraft don't even have HF radios fitted, and most pilots from basic PPL through to commercial pilots will go their entire careers without ever using HF.

VHF is more reliable, less prone to noise and distortion, has more channels available, uses simpler equipment, and all-round is always going to be the radio of choice. There's only one catch - it is a "straight line" transmission and is therefore limited in range by the horizon. That is not a problem in the populated parts of the world, but for operations in some oceanic airspace, or over some sparsely populated areas, you might be out of range of VHF stations. Only then will you resort to HF, which is why HF ops are a "specialist" kind of operation that most pilots will never need to bother with.

You may still need to answer an HF question in your exams, however

[framer]

Although you have received two posts on the benefits and technical aspects of HF, the abbreviated answer is we (almost) all use VHF all the time. Most aircraft don't even have HF radios fitted, and most pilots from basic PPL through to commercial pilots will go their entire careers without ever using HF.

I have to disagree with the above.

we (almost) all use VHF all the time.

If you are within range ,( ie line of sight) then you will use VHF, but a lot of flying involves crossing oceans and deserts. A lot of aircraft spend most of their flight out of range and unable to use VHF.

Most aircraft don't even have HF radios fitted

Of the 30 or 40 airliners I have flown every single one had two HF radios. About half of the G.A. aircraft I flew had HF radios and often on charters HF was all we had to communicate.

and most pilots from basic PPL through to commercial pilots will go their entire careers without ever using HF.

I can't refute that from personal experience but I'd be amazed if it was true. What do you base that on CJ?

[mutt]

and most pilots from basic PPL through to commercial pilots will go their entire careers without ever using HF.

I have to use it every flight as its part of our pre-flight checks

Mutt

[aterpster]

TWA had domestic airplanes and international airplanes. The domestic airplanes did not have HF sets.

A light airplane pilot in the U.S. is very unlikely to fly an airplane with HF sets.

[compressor stall]

Most aircraft don't even have HF radios fitted, and most pilots from basic PPL through to commercial pilots will go their entire careers without ever using HF.

In Europe most likely, and the lower 48 US, maybe. Definitely not a factual statement in most other parts of the world.

[Ercos]

Use VHF when the chart says to call on VHF and HF when the chart says to call on HF.

In all seriousness in most the world you use VHF, even on some Trans-Atlantic Routes (Blue Spruce Routes) you can maintain VHF communication the majority of the way. I've only used HF on trans-oceanic routes, over Darkest Africa, in the Canadian Arctic, over the Amazon, and over Siberia/Mongolia.

You will never use HF in a terminal area or in radar-controlled airspace, at least not in any of my experience.

[Dream Land]

Also, not previously mentioned, when using HF, you'r not speaking with a real ATC specialist, bUt rather a non controlling specialist who relays position reports and requests to the ARTCC which is responsible for aircraft separation.

Position reports must be in a standard format and as long as you receive a good SELCAL check, you don't have to continueously monitor the frequency. Someone with more technical knowledge can explain how the SELCAL check works.

Cheers, D.L.

[fireflybob]

Also HF is used via agencies such as Stockholm Radio for phone patch to Operations or even for a personal call!

Also useful for listening to the BBC World Service when overseas a long way from Blighty, especially on the longer sectors to help keep you awake.

And sometimes a good way to update weather by listening to the latest forecasts on Shannon etc for those that don't have the luxury of ACARS.

Charlie, Charlie?

[captjns]

HF is a very useful tool.

In 2001, we were flying a bunch of cruise ship passengers from Florida to Quebec with a drop off in Toronto. Our STA in Montréal-Mirabel International Airport was about 2200 local time.

Mirabel is in the middle of no-where. Great runways though! There is a hotel at the airport but our company was not the most compassionate when it came to short layovers. We had no idea as to what time the hotel closed.

Soooooo enter into the picture…. HF. Somewhere in the Carolinas, I had my F/E contact Miami Radio to establish a phone patch to the hotel. There were two cases of cold Canadian brew and trays of hot food awaiting our arrival at the Best Western in St. Germain later that evening.
It was a win win for all concerned. We supported the economy at the hotel. We filled our bellies after a long day of flying. Miami Radio increased their income by some $17.50 that day.
God bless HF. And I miss the Flight Engineer. Good job to the Best Western too.

[aterpster]

compressor stall:

In Europe most likely, and the lower 48 US, maybe. Definitely not a factual statement in most other parts of the world.

So, if I buy a new Cessna 172 for a trainer in England, or Spain, or such, it will have an HF radio?

As far as other than the lower-48 in the US, light aircraft in Hawaii don't have HF, and as far as I know they don't in Alaska either. In Alaska sat phones have become the backup for VHF.

Having said that control towers in the lower 48 transmitted on LF (below the AM band) as well as VHF well into the early 1960s. It was common in the 1930s and 1940s.

[Dream Land]

So, if I buy a new Cessna 172 for a trainer in England, or Spain, or such, it will have an HF radio?

No, mainly a tool for crossing vast expanses of water, where a line of site VHF (I know, not exactly) radio can't reach. You can rent HF radios in case you need to ferry.

Cheers, D.L.

[grounded27]

Nice to hear many reminiscing about HF experiences. Here in the US, large carrier, we only equip HF on aircraft that are required to have it by the governing factions. Hell we have SATCOM on just about everything but only temporarily install the HF equipment on an Airbus to fly it to Asia for heavy MX.

[con-pilot]

I have to use it every flight as its part of our pre-flight checks

You have a Rolex watch don't you? Every pilot that I flew with that had a Rolex watch checked their watch with the HF, frequency usually 15.000, every time they got into the cockpit and then reset their Rolex to the correct time. My Seiko Mickey Mouse watch never lost a second.

The answer to the question asked is, one uses HF when ATC tells you. The only time I was taken by surprise that I had to use HF, was over India the first time I flew over the country. VHF coasting in, but over the interior it was HF.

[aterpster]

con-pilot:

You have a Rolex watch don't you? Every pilot that I flew with that had a Rolex watch checked their watch with the HF, frequency usually 15.000, every time they got into the cockpit and then reset their Rolex to the correct time. My Seiko Mickey Mouse watch never lost a second.

I have an inexpensive Seiko that syncs with WWV every night.

[compressor stall]

Aterpster - what I meant by my admittedly very poorly worded reply above is that it is true that a pilot or aircraft in Europe would not normally use or be fitted with HF (the obvious exception being across the pond stuff), but the opposite in many other parts of the world (i.e. many many other countries have HF once you get beyond the cities).

[Denti]

I fly for a central european carrier and we do have HF on every aircraft, well, probably not on the Dash. Yes, we usually do not need it, but the odd flight to the azores or iceland requires its carriage depending on routing.

[cheemsaf]

A bit of a different answer:
In the fighter world -
HF: When talking to the guys on the ground BLOS, until we get close enough then we go to UHF. But most of the time its SINGARS or SATCOM. Otherwise its the tankers talking on HF when they're dragging us across an ocean somewhere.
VHF: within LOS to each other
UHF: within LOS to ATC or each other
When you're on the ground talking to other ground guys: HF via NVIS mostly (but you have to look up a bunch of space weather crap to figure out the best freqs to use given your location to location geometry and time of day, etc).